Plasmas, referred to as the “fourth state of matter,” are ionized gases having at least one electron that is not bound to an atom or molecule. In recent years, plasmas have become of significant interest to researchers in the fields such as organic and polymer chemistry, fuel conversion, hydrogen production, environmental chemistry, biology, and medicine, among others. This is, in part, because plasmas offer several advantages over traditional chemical processes. For example, plasmas can generate much higher temperatures and energy densities than conventional chemical technologies; plasmas are able to produce very high concentrations of energetic and chemically active species; and plasma systems can operate far from thermodynamic equilibrium, providing extremely high concentrations of chemically active species while having a bulk temperature as low as room temperature. Many details concerning the generation and applications of plasmas are described in PLASMA CHEMISTRY (2008), by Friedman.
Plasmas are generated by ionizing gases using any of the variety of ionization sources. Depending upon the ionization source and the extent of ionization, plasmas may be characterized as either thermal or non-thermal. Thermal and non-thermal plasmas can also be characterized by the temperature of their components. Thermal plasmas are in a state of thermal equilibrium, that is, the temperature of the free electrons, ions, and heavy neutral atoms are approximately the same. Non-thermal plasmas, or cold plasmas, are far from a state of thermal equilibrium; the temperature of the free electrons is much greater than the temperature of the ions and heavy neutral atoms within the plasma.
The initial generation of free electrons may vary depending upon the ionization source. With respect to both thermal and non-thermal ionization sources, electrons may be generated at the surface of the cathode due to a potential applied across the electrode. In addition, thermal plasma ionization sources may also generate electrons at the surface of a cathode as a result of the high temperature of the cathode (thermionic emissions) or high electric fields near the surface of the cathode (field emissions).
The energy from these free electrons may be transferred to additional plasma components, providing energy for additional ionization, excitation, dissociation, etc. With respect to non-thermal plasmas, the ionization process typically occurs by direct ionization through electron impact. Direct ionization occurs when an electron of high energy interacts with a valence electron of a neutral atom or molecule. If the energy of the electron is greater than the ionization potential of the valence electron, the valence electron escapes the electron cloud of the atom or molecule and becomes a free electron according to:e−+A→A++e−+e−.As the charge of the ion increases, the energy required to remove an additional electron also increases. Thus, the energy required to remove an additional electron from A+ is greater than the energy required to remove the first electron from A to form A+. A benefit of non-thermal plasmas is that because complete ionization does not occur, the power to the ionization source can be adjusted to increase or decrease ionization. This ability to adjust the ionization of the gas provides for a user to “tune” the plasma to their specific needs.
Although thermal plasmas are capable of delivering extremely high powers, they have several drawbacks. For example, thermal plasmas do not allow for adjusting the amount of ionization, they operate at extremely high temperatures, they lack efficiency, and may have electrode erosion problems.
Non-thermal plasma ionization sources have alleviated some of the above mentioned problems. Exemplary ionization sources for non-thermal plasmas include glow discharges, floating electrode dielectric barrier discharges (FE-DBD), and gilding arc discharges among others. In contrast to thermal plasmas, non-thermal plasmas provide for high selectivity, high energy efficiencies, and low operating temperatures. In many non-thermal plasma systems, electron temperatures are at about 10,000 K while the bulk gas temperature may be as cool as room temperature.
Dielectric barrier discharge (DBD) may be performed using an alternating current at a frequency of from about 0.5 kHz to about 500 kHz between a high voltage electrode and a ground electrode. In addition, one or more dielectric barriers are placed between the electrodes. DBDs have been employed for over a century and have been used for the generation of ozone in the purification of water, polymer treatment (to promote wettability, printability, adhesion), and for pollution control. DBDs prevent spark formation by limiting current between the electrodes.
Several materials can be utilized for the dielectric barrier. These include glass, quartz, and ceramics, among others. The clearance between the discharge gaps is typically between about 0.1 mm and several centimeters. The required voltage applied to the high voltage electrode varies depending upon the pressure and the clearance between discharge gaps. For a DBD at atmospheric pressure and a few millimeters between the gaps, the voltage required to generate a plasma is about 10 kV.
In certain embodiments, the ground electrode of the DBD may be an external conductive object, such as a human body. This is known as floating-electrode DBD (FE-DBD). FE-DBD has recently been utilized in medical applications.